1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device for displaying information or images by making use of external light such as illuminating light generated from an indoor illuminating lamp or natural light coming into a room from a window. More particularly, the present invention relates to a technique for increasing contrast of a reflection type liquid crystal display device.
2. Description of the Related Art
Normally white mode, TN-ECB, reflection type liquid crystal display devices are known. An example of such reflection type liquid crystal display devices is disclosed in, for example, Japanese Unexamined Patent Publication No. 4-116515. As illustrated in FIG. 5, a conventional TN-ECB, normally white mode, reflection type liquid crystal display device comprises a panel 0, a 1/4 wavelength plate 8, and a polarizing plate 70. The panel 0 comprises a transparent first substrate 1 disposed at the external light incoming side; a second substrate 2 opposing the first substrate 1, with the first substrate 1 and the second substrate 2 being put together so as to be separated by a predetermined distance; nematic liquid crystals 3 kept in the gap between both of the substrates, with the molecules thereof having a twisted orientation; and electrodes 7 and 10 for applying voltage to the nematic liquid crystals 3 at the first substrate 1 side and the nematic liquid crystals 3 at the second substrate 2 side. The electrode 7 formed at the first substrate 1 at the light-incoming side is transparent, while the electrode 10 formed at the second substrate 2 at the reflection side is capable of reflecting light. The polarizing plate 70 is disposed at the light-incoming side of the panel 0, with the 1/4 wavelength plate 8 being disposed between the polarizing plate 70 and the panel 0. When a voltage is not applied, the twisted orientation of the molecules of the nematic liquid crystals 3 is maintained, so that the nematic liquid crystals 3 act together as a 1/4 wavelength layer. Therefore, the nematic liquid crystals 3 cooperate with the polarizing plate 70 and the 1/4 wavelength plate 8 in order to pass external light and produce a white display. This is called normally white mode. When a voltage is applied, the orientation of the molecules of the nematic liquid crystals 3 changes to a substantially vertical orientation, so that the nematic liquid crystals 3 lose their ability to act together as a 1/4 wavelength layer. Therefore, the nematic liquid crystals 3 cooperate with the polarizing plate 70 and the 1/4 wavelength plate 8 in order to block external light in order to produce a black display.
FIG. 6 is a plan view schematically illustrating the optical arrangement of component parts of the reflection type liquid crystal display device of FIG. 5. In FIG. 6, the transmission axis of the polarizing plate 70 is designated 70P; the high refractive index orientation (optical anisotropic axis) of the 1/4 wavelength plate 8 is designated 8S; and the director (molecular major axis direction) of the liquid crystal molecules 4 at the first substrate 1 side is designated 4D. As is clear from FIG. 6, in the conventional reflection type liquid crystal display device, the optical anisotropic axis 8S of the 1/4 wavelength plate 8 is set at an angle of 45 degrees from the transmission axis 70P of the polarizing plate 70. Therefore, the linearly polarized light beams which pass through the polarizing plate 70 are converted into circularly polarized light beams by the 1/4 wavelength plate 8. The optical anisotropic axis 8S of the 1/4 wavelength plate 8 is perpendicular to the director 4D of the liquid crystal molecules 4 at the first substrate 1 side (or to the optical main axis of the nematic liquid crystals 3). When the optical main axis of the nematic liquid crystals 3 and the optical anisotropic axis 8S of the 1/4 wavelength plate (phase plate) 8 are set perpendicular to each other, the phase lead and the phase lag, which depend on the wavelength of the incident light, are cancelled, so that spectrum compensation of the reflected light is achieved. In other words, the optical relationship illustrated in FIG. 6 results in the most efficient polarization conversion in the panel 0, thereby increasing the white display reflectivity.
The conditions illustrated in FIG. 6 are set on the assumption that a simple 1/4 wavelength plate with retardations that become smaller with longer wavelengths is used. In addition, the aforementioned conditions are set only from the viewpoint of increasing white display reflectivity. However, 1/4 wavelength plates which are actually used are of the wide band type having substantially flat optical characteristics in the visible wavelength range. In general, the wide band type 1/4 wavelength plates have wavelength distribution characteristics with a slight peak in the green wavelength region and small retardations in the blue wavelength and green wavelength regions. Therefore, when a wide band 1/4 wavelength plate is used, the conditions of FIG. 6 are not necessarily appropriate. In addition, even when a n electrical field is applied and a black display is produced, the molecules of the nematic liquid crystals 3 do not actually stand up completely, so that there is still retardation. Therefore, a certain amount of light leaks when a black display is generated. Consequently, even when the liquid crystal display device is produced under conditions resulting in high white display reflectivity, the contrast (that is, the ratio between the white display reflectivity and the black display reflectivity) becomes very low, unless the black display reflectivity is made sufficiently low.